Ruby: Allow custom edges in API graph EntryPoints

ruby/ql/lib/codeql/ruby/ApiGraphs.qll

@@ -378,6 +378,10 @@ module API {
    *
    * Anything in the global scope is considered to be an entry point, but
    * additional entry points may be added by extending this class.
+   *
+   * By default, instances of this class will have a single edge from the root
+   * of the API graph. If you wish to construct more complex paths to an entry
+   * point, override the `edge` predicate.
    */
   abstract class EntryPoint extends string {
     bindingset[this]
@@ -400,6 +404,41 @@ module API {
 
     /** Gets an API-node for this entry point. */
     API::Node getANode() { result = root().getASuccessor(Label::entryPoint(this)) }
+
+    /**
+     * Holds if there is an edge from `pred` to this entry point, with label
+     * `lbl`. Override this predicate to define new paths to this entry point.
+     *
+     * For example, to define an entry point for `ActiveStorage::Attachment` we
+     * can use an intermediate entry point for `ActiveStorage`:
+     *
+     * ```ql
+     * class ActiveStorage extends EntryPoint {
+     *   ActiveStorage() { this = "ActiveStorage" }
+     *
+     *   override predicate edge(Node pred, Label::ApiLabel lbl) {
+     *     pred = root() and lbl = Label::member("ActiveStorage")
+     *   }
+     * }
+     *
+     * class Attachment extends EntryPoint {
+     *   Attachment() { this = "ActiveStorage::Attachment" }
+     *
+     *   override predicate edge(Node pred, Label::ApiLabel lbl) {
+     *     pred = getTopLevelMember("ActiveStorage") and
+     *     lbl = Label::member("Attachment")
+     *   }
+     *
+     *   override DataFlow::LocalSourceNode getAUse() { result = customAttachmentPredicate() }
+     * }
+     * ```
+     *
+     * This means that
+     * `getTopLevelMember("ActiveStorage").getMember("Attachment")` will return
+     * results from `customAttachmentPredicate()`, even if there are no
+     * references to `ActiveStorage` or `Attachment` in the codebase.
+     */
+    predicate edge(API::Node pred, Label::ApiLabel lbl) { none() }
   }
 
   // Ensure all entry points are imported from ApiGraphs.qll
@@ -718,6 +757,8 @@ module API {
       exists(EntryPoint entry |
         pred = root() and
         lbl = Label::entryPoint(entry)
+        or
+        entry.edge(pred, lbl)
       |
         succ = MkDef(entry.getASink())
         or